Recently, nanometer-size particles (nanoparticles) with an average diameter of less than 1 μm are applied to various technical fields. In the field of pharmaceuticals, it is expected that the nanometer-size particles provide advantageous effects.
As an example of the advantageous effects, a poorly water-soluble or water-insoluble medicinal ingredient is converted into nanoparticles. It is found that intravenous injection of the nanoparticles dispersed in water does not form thrombus. In addition, the injection may enhance medicinal ingredient absorption in vivo. Therefore, it is expected that the medicinal nanoparticles are utilized in a drug delivery system (DDS), in which a pharmaceutical agent or gene is delivered to an affected part, as an effective measure for cancer therapy or gene therapy.
Conventionally, few methods for preparing the medicinal nanoparticles of a drug have been known. The conventional methods include, for example, a wet grinding method (refer to Patent Reference 1), in which a wet mill is used to grind a medicinal ingredient in presence of a surfactant agent, so that the medicinal ingredient is converted into nanoparticles; a high pressure homogenization method (refer to Patent Reference 2), in which a high pressure is applied to homogenize a medicinal ingredient for producing nanoparticles of the medical ingredient; and a fluid bed spray-drying method (refer to Patent Reference 3), in which an organic solution of a poorly water-soluble or water-insoluble compound is sprayed to a fluidized bed of carrier excipient particles under a condition that an organic solvent is removed from the organic solution, thereby obtaining a mixture of the medicinal nanoparticles and the carrier excipient particles.
However, there are several problems in the conventional methods mentioned above.
In the first method, or the wet grinding method, metal impurities coming from a metal ball for the grinding process tend to mix in the medicinal ingredient. Therefore, it is difficult to obtain a suspension exclusively comprising the medicinal nanoparticles.
The second method, or the high pressure homogenization method, is used to minimize a size of liquid globules of emulsion and liposome, and its applicability to a solid substance depends on a physical property of the substance. Therefore, the method is applicable only to limited types of medicinal ingredients.
When the third method, or the fluid bed spray-drying method, is used, the organic solvent may remain in the obtained mixture. Therefore, it is difficult to obtain a suspension exclusively comprising the medicinal nanoparticles.
As described above, with the conventional methods, it is possible to convert the medicinal ingredient into the medicinal nanoparticles. However, in order to safely administrate the medicinal nanoparticles to a human body, several critical problems have to be solved.    [Patent Reference 1] U.S. Pat. No. 5,145,684    [Patent Reference 1] U.S. Pat. No. 5,510,118    [Patent Reference 1] Japanese Patent Publication No. 2003-518038
In order to overcome these problems in the prior art, the present invention provides a method of obtaining a medicinal nanoparticle suspension suitable for administration to a human body. The method of the present invention makes it possible to prepare nanopaticles of a poorly water-soluble or water-insoluble medicinal ingredient while preventing impurity from contaminating.